Underground utility lines may emit passive electromagnetic signals for various reasons. An electric utility, for example, carries its own electric signal at 50–60 Hz. Other utility lines emit electromagnetic fields within a frequency range of 13 to 22 kHz responsively to ambient fields that induce signals on the utility. Gas providers typically place 100–120 Hz signals on gas lines to provide cathodic protection against corrosion. CATV lines typically carry signals at about 31 kHz. As used herein, any electromagnetic field emitted by a utility line, whether generated by signals carried directly by the utility or by signals induced on the utility by signals ambient to the utility, where the signal is not applied by a cable location system for the purpose of locating the cable, is considered a “passive” signal.
It is known to provide an above-ground portable locator having an antenna tuned to receive signals within a desired frequency range for locating the underground utility. Typically, the operator has a general idea of the underground utility's location. If so, the operator carries the locator so that the antenna is parallel to the ground and perpendicular to the expected line of the utility and walks generally perpendicular to the expected line. The operator views a signal intensity display on the locator and finds the position at which a peak signal appears. At that point, the above-ground position may be marked and a depth reading taken. Where the operator does not know the utility's general location, he may find the utility by walking in a grid pattern while carrying the locator so that the antenna is parallel to the ground. The operator first walks in parallel lines over the area. If he does not find a peak value, he may be carrying the antenna parallel to the utility. Since underground lines emit a generally cylindrical magnetic field, the magnetic flux lines are perpendicular to the coil antenna's axis in this orientation and do not induce a significant signal in the antenna. Thus, the operator then turns 90° and walks in parallel paths over the same area. In such a pattern, the antenna is more likely to be perpendicular to the utility and therefore more likely to detect a peak signal. Of course, the operator may not be exactly parallel or exactly perpendicular to the utility in either direction. By using the grid pattern, however, the operator should be able to cross over the utility with the antenna at such an angle that a peak can de detected. At such a point, the operator may rotate the locator about a vertical axis until the antenna is perpendicular to the utility and a maximum signal is measured. Thus, the locator's orientation notifies the operator of the utility's direction. An example of a system capable of locating existing underground utility lines carrying active or passive signals is described in U.S. Pat. No. 6,102,136, the entire disclosure of which is incorporated by reference herein.
FIG. 1 schematically illustrates components of a portable above-ground locator 10 used to detect passive signals within a frequency range of 13 to 22 kHz resulting from ambient radio frequency signals. A coil antenna 12 receives electromagnetic signals emitted by the utility line, generates a signal responsively thereto and outputs the signal to an amplifier 14 and a pair of bandpass filters 16 and 18. Filter 16 has a bandpass range of 13 to 17 kHz, while filter 18 passes a range of 18 to 22 kHz. Two bandpass filters are used, as opposed to a single filter of larger bandwidth, to improve the signal-to-noise ratio. A multiplexer 20 selects the output of either of the two filters, depending on instructions received from a CPU 22 controlled by the operator through a keyboard at the locator display. Multiplexer 20 directs the signal from the selected filter to an analog-to-digital converter 24, and the resulting digital signal passes to CPU 22. The locator includes a display 26 that indicates the strength of the signal detected by antenna 12 so that the operator may locate a signal peak and, therefore, a point above the underground utility.
The locator may also be used to locate signals in other frequency ranges. Generally, the locator includes low pass and bandpass filters, which are configured to pass the respectively desired frequency ranges, in parallel with filters 16 and 18. The CPU selects the filter for a desired frequency range through a multiplexer.
In one known locator, three antennas are used to determine location and depth of underground utilities. A locator housing (not shown) includes three parallel horizontally-aligned antennas (not shown). The bottom antenna (hereafter “C1”) is disposed in the housing so that it is parallel to the ground when the operator carries the locator during operation. The second antenna (hereafter “C2”) is directly above the first antenna and, being parallel to antenna C1, is also parallel to the ground. The third parallel antenna (hereafter “C3”) is directly above antenna C2 in the housing a distance equal to the distance between antennas C1 and C2. The distance between C1 and C3 is referred to herein as “d.” The three antennas are connected to amplifier 14 through a multiplexer (not shown) controlled by the CPU.
In operation, the CPU repeatedly samples the signals on the three antennas and relies on the difference in signal strength between C1 and C3 (C1−C3=E1) and between C2 and C3 (C2−C3=E2) to locate an underground utility and determine its depth. In locating an underground utility, the CPU monitors and displays the value of E1, and the operator finds a point above the utility by finding the peak value of E1 in a manner as described above. Once finding such a position, the operator activates a button on the locator to determine the utility's depth, which the CPU calculates according to the function: depth=(d)(E2 )/(E1−(2(E2))).